Genes and traits

A gene is a section of DNA that carries instructions for something an organism builds or does. Some genes code for proteins that build eye pigment, enzymes, or body parts. Together the genes are the recipe book of life. When a new baby is made, the recipe book is assembled by taking one copy of every gene from each parent.

This is why children resemble both parents, not just one. From each parent they receive one allele of each gene, and alleles are simply different versions of the same gene. A brown-eye allele and a blue-eye allele are two versions of one gene. The two versions pair up, and the combination helps decide the trait you see. Every person carries a mix drawn from both sides of the family.

Some alleles are dominant: they show up even when paired with a different version. Others are recessive: they show only when a child receives two copies of them. This is why two brown-eyed parents can both carry a hidden blue-eye allele and have a blue-eyed child. Dominant and recessive describe how the versions interact; they say nothing about which version is healthier or stronger.

Now the weightlifter's child. Strong muscles built by years of training are an acquired trait: they came from exercise and diet, and the genes did not change. Only mutations written into the DNA of the germ cells can be inherited. Able to lift more, speak another language, or run faster: all of that is learned anew by her children. Generations of blacksmiths do not automatically give birth to giant babies.